THE GEOMORPHIC SIGNATURE OF ACTIVE TECTONIC SHEAR ALONG THE WALKER LANE/BASIN AND RANGE TRANSITION ZONE

The Walker Lane tectonic belt represents the boundary between the Sierra Nevada and Basin and Range provinces and accommodates up to 10-25% of the Pacific/North America relative plate motion (shear). Geodetic observations as well as historical earthquakes in the Central Nevada Seismic Belt that exhibit both normal and dextral displacements indicate that Pacific/North American relative shear extends over 200 km into the Basin and Range. The physiographic expression of mountain ranges in central Nevada rotate from north-south orientations in the east to progressively more northeasterly trends to the west. However, paleoseismic observations of dextral shear in the western Basin and Range are few likely a reflection of slow slip rates, long recurrence intervals, and small displacements that lead to low preservation of dextral offsets. Here I present physiographic observations of late Pleistocene ruptures and paleoseismic observations from studies along the Petersen Mountain, Bonham Ranch, and San Emidio faults. The observations support a component of dextral slip east of the Walker Lane and include (1) left-stepping patterns of ruptures that project 10-20 degrees off of rangefronts and across alluvial surfaces, (2) ruptures that extend across valley floors that are not associated with range fronts, (3) stratigraphic juxtapositions in trench exposures, and (4) tectonic geomorphic features. I infer that the pattern of latest Pleistocene ruptures are responding to an evolving stress field related to the northward migration of the Mendocino Triple Junction which passed through the latitude of the northern Walker Lane over the last several million years. The geomorphic signature of active tectonic deformation may be related to an overprinting of shear on previous extensional structures and the creation of new fault linkages to more efficiently accommodate shear.